Although the physiological importance of cholecystokinin (CCK) and the enteropancreatic reflex in regulating pancreatic enzyme secretion has been established, little is known about the feedback regulation of these mechanisms. Previously we demonstrated that trypsin in the intestine suppresses the release of CCK and pancreatic enzyme secretion and that this is mediated by a trypsin-sensitive peptide which we named "CCK- releasing peptide." This peptide is secreted into the proximal small intestine to stimulate the release of CCK. When trypsin is present this peptide is cleaved and inactivated. Our studies also showed that although trypsin supresses CCK release, it has no effect on pancreatic secretion stimulated by the enteropancreatic neural reflexes. This reflex appears to be modulated by pancreatic polypeptide (PP) which suppresses pancreatic enzyme secretion by inhibiting cholinergic transmission. The aims of this proposal are to investigate the biochemistry and physiology of the CCK-releasing peptide and establish its role in the mediation of the postprandial release of CCK. Furthermore we also plan to investigate the cellular mechanism(s) by which PP inhibits cholinergic transmission in the pancreas. Toward these goals we plan to continue to isolate and purify the CCK-releasing peptide further to homogeneity so that it will be sequenced and synthesized. Polyclonal antibodies can be raised against this peptide to develop radioimmunoassays for characterization of tissue distribution and molecular forms of CCK-releasing peptide in the GI tract and sulcus entericus. We also plan to investigate the effects of neural, endocrine and luminal factors on the secretion of CCK releasing peptide and establish the pysiological role of this newly discovered peptide in mediating postprandial release of CCK. In part II of this study in vitro autoradiography will be used to identify PP receptors on pancreatic cholinergic nerves. We will also examine the signal transduction mechanisms mediating the inhibitory action of PP on cholinergic transmission. The techniques of intracellular recording will be applied to characterize the effects of PP on the electrophysiologic properties of pancreatic neurons. Through these studies we hope to gain insight into the mechanisms mediating feedback regulation of pancreatic enzyme secretion and improve our understanding of neuroendocrine control of pancreatic function.